It is known that the RNP concept corresponds to area navigation, for which are added (on board the aircraft) monitoring and warnings which make it possible to ensure that the aircraft remains in a corridor, called RNP, around a reference trajectory and which authorize the taking into account of curved trajectories. Outside of this corridor, there is potentially relief or other aircraft. The required performance for an operation of the RNP type is defined by an RNP value which represents the half-width (in nautical miles: NM) of the corridor around the reference trajectory, within which the aircraft must remain for 95% of the time during the operation. A second corridor (around the reference trajectory) having a half-width of twice the RNP value is also defined.
The concept of RNP AR operations is yet more restrictive. The RNP AR procedures are in fact characterized by:                RNP values:        which are less than or equal to 0.3 NM on approach, and which can drop to 0.1 NM; and        which are strictly less than 1 NM on departure and during a go-around, and which can also drop to 0.1 NM;        a final approach segment which can be curved; and        obstacles (mountains, traffic . . . ) which can be situated at twice the RNP value with respect to the reference trajectory whereas, for the usual RNP operations, an additional margin with respect to obstacles is provided.        
The air authorities have defined a target level of safety TLS (Target Level of Safety) of 10−7 per operation, whatever its type may be. In the case of RNP AR operations, as the RNP values can drop to 0.1 NM and the obstacles can be situated at twice the RNP value from the reference trajectory, this target is represented by a probability that the aircraft exits from the corridor of half-width D=2.RNP which must not exceed 10−7 per procedure.
In order to meet these requirements, the automated flight management system of the aircraft uses architectures and monitoring which makes it possible to attain the safety level set by the air authorities. These specific architectures and monitoring are not used in the flight phases outside of RNP AR operations and must therefore be activated at the opportune time in order to make it possible to carry out the RNP AR operation in total safety.
Moreover, RNP AR operations can be constructed on the basis of several possible RNP levels, allowing increasingly low landing minimums. The crew must choose the RNP value to be used according to the capabilities of the aircraft and of the crew, and to current meteorological conditions. This value is defined by the crew in the flight management system at the moment when the operation must start.
Now, the activation of the architectures and monitoring specific to the RNP AR operations, although automatic, is initiated only when the aircraft is engaged in this operation. This late activation does not allow the crew to evaluate the real capabilities of the aircraft sufficiently upstream of the engagement of the operation and therefore does not allow the use of an alternative strategy if necessary (such as carrying out another type of approach for example).